This invention relates generally to monolithic series connected solar cells, and more particularly the invention relates to electrically isolating adjacent cells in such a structure.
The semiconductor solar cell comprises a plurality of p and n conductivity-type regions in a semiconductor body which generate a voltage potential and/or a current when electron-hole pairs are created in the semiconductor body in response to impinging radiation and the holes and electrons migrate to the p-doped region and n-doped region, respectively.
Silicon solar cells generally operate at high currents and low voltages. For example, a solar cell fabricated on a standard 4 inch round wafer will produce about 3 amperes of current at a voltage of about 0.6 volt. These high currents (e.g. 3 amperes for only 1.8 watts of power) make it difficult to interconnect cells into any module without voltage drops comparable to the 0.6 volt output voltage, thus resulting in very significant power losses. For solar cells designed to work under concentrated light, the problem is much more severe. For example, at 300.times. concentration, the current from such a wafer would be about 900 amperes, yet the voltage would be only 0.7 volt. For applications requiring large, high power solar cells many cells on the same wafer are series connected so that the output voltage from the wafer can be multiplied. Accordingly, in monolithic structures where the cells share a common substrate, electrical isolation of the cells must be provided.
Heretofore a number of monolithic solar cell configurations have been provided for serially connecting the individual cells. Borden U.S. Pat. No. 4,278,431 proposes a mesa structure for each cell with the mesa physically separating epitaxial layer regions which comprise the active portions of each cell. The cells are then serially connected by metal plating formed over oxide isolation in the grooved regions between mesas. Goetzberger U.S. Pat. No. 4,330,680 uses physical shaping to increase electrical resistance of a semiconductor substrate between cells and thereby provide enhanced electrical isolation. P and n regions for each cell are formed on opposing sides of the substrate, and the grooves are chemically etched from alternate sides of the substrate to increase resistance of the substrate between cells. Swanson U.S. Pat. No. 4,933,021 discloses an interdigitated solar cell having p and n regions formed in alternating rows in one surface of a substrate with metal contacts provided and contacting all of the doped regions in one row with all rows of like doped regions being connected in parallel. Electrical isolation between cells in the interdigitated solar cell is enhanced by the use of shorted p-n junctions between cells with metallization for serially connecting the cells, shorting the p-n junctions, and thereby absorbing minority carriers flowing between the adjacent cells.
Kaplow et al. U.S. Pat. No. 4,128,732 and U.S. Pat. No. 4,193,081 disclose solar cells which are individually isolated in a substrate by etching completely through the substrate and with the removed portions in the substrate also providing cooling passages between adjacent side walls of adjoining unit cells through which a heat transfer medium is transmitted.